Ethylene Sensor

ABSTRACT

A sensor device can include a transition metal complex capable of interacting with a carbon-carbon multiple bond moiety. The sensor can detect the fruit-ripening hormone ethylene with high sensitivity.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Application No.61/614,834, filed Mar. 23, 2012, which is incorporated by reference inits entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.W911NF-07-D-0004 awarded by the Army Research Office. The government hascertain rights in this invention.

TECHNICAL FIELD

This invention relates to ethylene sensors, materials for use inethylene sensors, and methods of making and using these.

BACKGROUND

Ethylene, the smallest plant hormone, plays a role in many developmentalprocesses in plants. It initiates the ripening of fruit, promotes seedgermination and flowering, and is responsible for the senescence ofleaves and flowers. As fruits and vegetables start ripening, ethylene isproduced and emitted, and the internal ethylene concentration in somefruits is used as a maturity index to determine the time of harvest. Insome vegetables and fruits, such as bananas, exposure to ethylene gasresults in a continuation of the ripening process after harvesting, thusthe monitoring of ethylene gas in storage rooms is important to avoidthe deterioration of ethylene sensitive produce.

SUMMARY

A reversible chemoresistive sensor able to detect sub-ppm concentrationsof analytes such as ethylene is described. The ethylene-responsivematerial has high selectivity towards ethylene and is prepared simply infew steps from commercially available materials. The sensing mechanismcan take advantage of the high sensitivity in resistance ofsingle-walled carbon nanotubes (SWCNTs or SWNTs) to changes in theirelectronic surroundings, and the binding of a copper(I) complex tocarbon-carbon multiple bonds.

In one aspect, a sensor includes a conductive material comprising acarbon-carbon multiple bond moiety, the conductive material being inelectrical communication with at least two electrodes; and a transitionmetal complex capable of interacting with a carbon-carbon multiple bondmoiety.

The conductive material can include a carbon nanotube. The transitionmetal complex can be capable of forming a stable complex with ethylene.The transition metal complex can be associated with the carbon nanotubeby coordination of the transition metal to the carbon-carbon multiplebond moiety. The transition metal complex can be associated with thecarbon nanotube by a covalent link between the carbon nanotube and aligand of the transition metal complex. The transition metal complex canbe associated with the carbon nanotube by a polymer which isnon-covalently associated with the carbon nanotube. The transition metalcomplex can be bound to the carbon-carbon multiple bond moiety of theconductive material.

The transition metal complex can have formula (I):

where:

M can be a transition metal; each R¹, independently, can be H, halo,alkyl, or haloalkyl; each R², independently, can be H, halo, alkyl,haloalkyl, or aryl; R³ can be H or alkyl; and L can be absent orrepresent a ligand.

The transition metal complex can have formula (II):

where:

M can be a transition metal; each R⁴, independently, can be alkyl,haloalkyl, aryl, or trialkylsilyl; A can be —CH(R⁵)—X—CH(R⁵)— wherein Xis N or CH, and each R⁵, independently, can be H, halo, alkyl, orhaloalkyl; or A can be —P(R⁶)₂—, wherein each R⁶, independently, isalkyl; and L can be absent or represent a ligand.

The transition metal complex can have the formula:

where:

each R¹, independently, can be H, methyl, or trifluoromethyl; each R²,independently, can be H, methyl, trifluoromethyl, or phenyl; R³ can be Hor methyl; and L can be absent, a thiol, or a carbon-carbon multiplebond.

In another aspect, a method of sensing an analyte includes exposing asensor to a sample, the sensor including: a conductive materialcomprising a carbon-carbon multiple bond moiety, the conductive materialbeing in electrical communication with at least two electrodes; and atransition metal complex capable of interacting with a carbon-carbonmultiple bond moiety; and measuring an electrical property at theelectrodes.

The sample can be a gas. The electrical property can be resistance orconductance. The analyte can be ethylene. The conductive material caninclude a carbon nanotube. The transition metal complex can be capableof forming a stable complex with ethylene. The transition metal complexcan be associated with the carbon nanotube by coordination of thetransition metal to the carbon-carbon multiple bond moiety. Thetransition metal complex can be associated with the carbon nanotube by acovalent link between the carbon nanotube and a ligand of the transitionmetal complex. The transition metal complex can be associated with thecarbon nanotube by a polymer which is non-covalently associated with thecarbon nanotube. The transition metal complex can be bound to thecarbon-carbon multiple bond moiety of the conductive material.

In the method, the transition metal complex can have formula (I) orformula (II) as described above. In the method, the transition metalcomplex can have the formula:

where:

each R¹, independently, can be H, methyl, or trifluoromethyl; each R²,independently, can be H, methyl, trifluoromethyl, or phenyl; R³ can be Hor methyl; and L can be absent, a thiol, or a carbon-carbon multiplebond.

A composite including the transition metal complex and the carbon-carbonmultiple bond moiety, for example, SWCNT, can be mixed with a polymer,for example, in the form of polystyrene beads.

In another aspect, a method of making a sensor includes forming acomplex including a conductive material comprising a carbon-carbonmultiple bond moiety, and a transition metal complex capable ofinteracting with a carbon-carbon multiple bond moiety; and placing theconductive material in electrical communication with at least twoelectrodes.

In another aspect, a method of making of making a sensor includesforming a complex including a conductive material comprising acarbon-carbon multiple bond moiety, a transition metal complex capableof interacting with a carbon-carbon multiple bon moiety, and one or morepolymers; and placing the conductive material in electricalcommunication with at least two electrodes.

The method can include spray drying the complex at a temperature toobtain a viscous conductive material, and the viscous material can beplaced in electrical communication with at least two electrodes. Thetemperature can be between 100 and 210° C., 140° C. and 210° C., 180° C.to 210° C., for example, above 200° C. The spray drying can take placein an inert atmosphere, such as nitrogen.

The transition metal can be copper. The electrodes can be gold. Thesensor can be configured to sense ethylene. The complex can be a Cu(I)scorpionate. The complex can be Cu(I) scorpionate 1.

The placing of the conductive material can include applying theconductive material and one or more polymers onto at least twoelectrodes by drop-casting, spin-coating, screen-printing, inkjetprinting, spreading, painting, or pelletizing and abrading the materialonto a surface, or combinations thereof. The conductive material andpolymer (or polymers) can be applied simultaneously or in sequence.

In some embodiments, the polymer can be a hydrophobic polymer such aspolyethylene or polystyrene. In some embodiments, the polymer can be afluorinated polymer, which can be partially fluorinated orperfluorinated (e.g. polyvinylidene fluoride, Nafion). In someembodiments, the polymer can contain ionic groups (e.g. Nafion). In someembodiments, the polymer can be conjugated or partially conjugatedpolymers including polyacetylene, polyphenylenevinylene, polythiophene,polypyrrole or polyaniline, optionally including electron donatinggroups, such as alkoxy groups (e.g.Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]). In someembodiments, the polymer can be a mixture of polymers, includingconjugated or nonconjugated mixtures or copolymers.

In some embodiments, the polymer can be selected from the groupconsisting of a polyethylene, a polystyrene, a poly(ethylene oxide), apolyvinylidene fluoride, a Nafion, a polyphenylenevinylene, andcombinations thereof.

In some embodiments, the method can include combining the complexmixture with a selector, such as a transition metal salt, for example,Ag(OTf) or Pd(OCOCF₃)₂.

Other aspects, embodiments, and features will be apparent from thefollowing description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of ethylene detection by achemoresistive sensor. A mixture of single-walled carbon nanotubes(SWCNTs or SWNTs) and copper complex 1 is drop-cast between goldelectrodes, and the change in resistance in response to ethyleneexposure is measured. The copper complexes partly bind to ethylenemolecules, forming ethylene complex 2, and resulting in a resistancechange.

FIG. 2 a is a schematic illustration of an experimental setup forsensing measurements. A continuous gas flow is directed through thedevice chamber. The gas stream can be switched between nitrogen gas(“Zero” mode) or the nitrogen gas analyte mixture (“Span” mode), inwhich the gas stream runs through the flow chamber containing theanalyte (ethylene) or a piece of fruit. FIG. 2 b is a schematicillustration of a gas flow chamber.

FIG. 3 a shows relative responses of 1-SWCNT devices to 0.5, 1, 2, 5,20, and 50 ppm ethylene diluted with nitrogen gas and of pristine SWCNTto 20 ppm ethylene (the inset shows the responses of 1-SWCNT to 0.5, 1,and 2 ppm and of SWCNT to 20 ppm. FIG. 3 b shows average responses fromthree different devices each. FIG. 3 c shows a plot of average responsevs. ethylene concentration.

FIG. 4 shows the optimized structure of 3, in which 1 is coordinativelybound to a (6,5) SWCNT fragment (B3LYP/6-31G*, LanL2DZ for Cu; hydrogenatoms at the ends of the SWCNT fragment and on the pyrazol rings havebeen omitted for clarity).

FIG. 5, top: Raman spectra of 1-SWCNT and pristine SWCNT (dashed line;laser energy 785 nm); bottom: IR spectrum of 1-SWNT.

FIG. 6 a. shows responses of 1-SWCNT devices to 100 g of different fruitrelative to 20 ppm ethylene; FIG. 6 b shows responses to fruit monitoredover several weeks.

FIG. 7 shows relative responses of 1-SWCNT devices and pristine SWCNT to50 ppm ethylene and various solvents diluted with nitrogen gas(respective concentrations are given in parentheses in ppm).

FIG. 8 shows a comparison of the responses of 1-SWCNT devices and1-PS-SWCNT devices to 0.5, 1, and 2 ppm ethylene.

FIGS. 9 a-9 d are graphs showing results of PET measurements: FIG. 9 a,source-drain current for pristine SWCNTs; FIG. 9 b, gate leakage currentfor pristine SWCNTs; FIG. 9 c, source-drain current for 1-SWCNT; andFIG. 9 d, gate leakage current for 1-SWCNT. The voltage was swept from 0to +2 V to −20V.

FIG. 10 shows responses to 20 ppm ethylene of (left) 1-SWCNT devicesmade from different types of SWCNTs and (right) devices made from1-SWCNT, 2-SWCNT, SWCNTs with [Cu(CH₃CN)₄]PF₆ and 4-SWCNT.

FIG. 11 shows results of XPS measurements: FIG. 11 a, survey scans of 1,2, and 1-SWCNT; and FIG. 11 b, high resolution scans of the Cu 2p regionof 1, 2, and 1-SWCNT.

FIG. 12 is a schematic illustration of ethylene sensing usingpolymer-wrapped SWCNTs.

FIG. 13 shows the response of a PT1/SWCNT/1 device to ethylene.

FIG. 14 is a schematic illustration of ethylene sensing using covalentlymodified SWCNTs.

FIG. 15 is a schematic illustration of covalent modification of SWCNTs.

FIG. 16 shows the response of a device having covalently modified SWCNTsto ethylene.

FIG. 17 is a graph showing the response of sensor fabricated by drawingwith a pellet of Cu(I) scorpionate ethylene complex 2 and SWCNTs toethylene.

FIG. 18 is a graph showing the response of sensor fabricated by drawingwith a pellet of Cu(I) scorpionate ethylene complex 2 and SWCNTs toethylene.

FIG. 19 is a graph showing the response of sensor fabricated by drawingwith a pellet of SWCNTs to ethylene.

FIG. 20 is a graph showing the sensing response of devices fabricated byabrasion with pristine SWCNTs, SWCNTs+KMnO₄ and SWCNTs+1 on HPmultipurpose paper to 500 ppm ethylene.

FIG. 21 is a graph showing the sensing response of devices fabricated byabrasion with SWCNTs+1 and pristine SWCNTs on the surface of weighingpaper to 40 ppm ethylene.

FIG. 22 is a graph showing the sensing response of devices based on1-SWCNTs (spray dried) and pristine SWCNTs to 20 ppm and 10 ppmethylene. Dashed lines indicated the time at which the exposure wasstarted. Devices were exposed to ethylene for 30 sec each.

FIG. 23 is a photograph of a glass slide with 14 devices that werecoated with different polymers.

FIG. 24 is a graph showing the sensing response of devices based on1-SWCNTs (spray dried), Ag(OTf)-SWCNT, Pd(OCOCF₃)₂—SWCNTs and twodifferent types of pristine SWCNTs to 20 ppm and 10 ppm ethylene. Dashedlines indicated the time at which the exposure was started. Devices wereexposed to ethylene for 30 sec each.

FIG. 25 is a graph showing the sensing response of devices based on1-SWCNTs (spray dried), Ag(OTf)-SWCNT, Pd(OCOCF₃)₂—SWCNTs and twodifferent types of pristine SWCNTs to 2700 ppm and 1350 ppmtetrahydrofuran. Dashed lines indicated the time at which the exposurewas started. Devices were exposed to ethylene for 30 sec each.

DETAILED DESCRIPTION

Because of its small size and lack of polar chemical functionality,ethylene is generally hard to detect. Traditionally, gas chromatographyand photoacoustic spectroscopy have been used to measure ethyleneconcentrations. See, for example, H. Pham-Tuan, et al., J. Chromatogr. A2000, 868, 249-259; and M. Scotoni, et al., Appl. Phys. B 2006, 82,495-500; each of which is incorporated by reference in its entirety.Both techniques suffer from the disadvantage of being operationallyimpractical and do not allow for real-time measurements. Other sensingsystems that have been suggested use electrochemical or chemoresistivemethods, magnetoelastic sensing, photoluminescence quenching, andfluorescence turn-on. All of these systems have drawbacks such as highcost, impracticability, or insufficient sensitivity towards ethylene.See, e.g., L. R. Jordan, et al., Analyst 1997, 122, 811-814; Y.Pimtong-Ngam, et al., Sens. Actuators A 2007, 139, 7-11; M. A. G.Zevenbergen, et al., Anal. Chem. 2011, 83, 6300-6307; R. Mang, et al.,Sensors 2002, 2, 331-338; 0. Green, et al., J. Am. Chem. Soc. 2004, 126,5952-5953; and B. Esser, et al., Angew. Chem. Int. Ed. 2010, 49,8872-8875; each of which is incorporated by reference in its entirety.In addition, gas-sampling tubes based on a colorimetric reaction areavailable (see A. A. Kader, M. S. Reid, J. F. Thompson, in PostharvestTechnology of Horticultural Crops, (Ed: A. A. Kader), University ofCalifornia, Agricultural and Natural Resources, Publication 3311, 2002,pp. 39 ff., 55 ff., 113 ff., 149 ff., 163 ff, which is incorporated byreference in its entirety).

In general, a sensor (e.g., a chemoresistive or FET sensor) includes aconductive material including a carbon-carbon multiple bond moiety, theconductive material being in electrical communication with at least twoelectrodes; and a transition metal complex capable of interacting with acarbon-carbon multiple bond moiety. A measurable property of the sensor(e.g. resistance, conductivity, or other electrical property measuredbetween electrodes) changes upon exposure of the sensor to an analyte.The transition metal complex can be mixed with a particulate material,such as polymer beads (e.g., polystyrene beads) or other material toincrease the surface area of an active sensing region of the sensor orto exploit the potential of the particulate material to act as apreconcentrator for the analyte. The sensor can be an element of anarray sensor that can include one or more of the sensor including aconductive material including a carbon-carbon multiple bond moiety. Forexample, the array can include 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,14, 15, 16, 17, 18, 19, 20, 25, 50, or more sensor elements.

The analyte can have an electron-rich moiety capable of interacting withthe transition metal complex. For example, the electron-rich moiety caninclude a carbon-carbon multiple bond; a carbon-nitrogen multiple bond;or a lone pair of electrons (e.g., as may be found in the C═O moiety ofan aldehyde or ketone). In some cases, the analyte includes acarbon-carbon double bond, such as is found in ethylene, propylene, andother alkenes; or the analyte can include a carbon-carbon triple bond,such as is found in acetylene, propyne, or other alkynes.

The conductive material can include a conductive carbon-containingmaterial, including but not limited to carbon nanotubes, conductivepolymers, or combinations thereof, and further including additionalcomponents such as other polymers, binders, fillers, or the like. Theconductive carbon-containing material can include, for example, SWCNTs,MWNTs, conductive polymers such as a poly(acetylene), a poly(phenylenevinylene), a poly(pyrrole), a poly(thiophene), a poly(aniline), apoly(phenylene sulfide), or other conductive polymers, or combinationsthereof. A conductive polymer can include a copolymer or mixtures ofpolymers. The conductive polymer can include a carbon-carbon multiplebond moiety.

Polymers or polymer materials, as used herein, refer to extendedmolecular structures comprising a backbone (e.g., non-conjugatedbackbone, conjugated backbone) which optionally contain pendant sidegroups, where “backbone” refers to the longest continuous bond pathwayof the polymer. In some embodiments, the polymer is substantiallynon-conjugated or has a non-conjugated backbone. In some embodiments, atleast a portion of the polymer is conjugated, i.e. the polymer has atleast one portion along which electron density or electronic charge canbe conducted, where the electronic charge is referred to as being“delocalized.” A polymer may be “pi-conjugated,” where atoms of thebackbone include p-orbitals participating in conjugation and havesufficient overlap with adjacent conjugated p-orbitals. It should beunderstood that other types of conjugated polymers may be used, such assigma-conjugated polymers.

The polymer can be a homo-polymer or a co-polymer such as a randomco-polymer or a block co-polymer. In one embodiment, the polymer is ablock co-polymer. The polymer compositions can vary continuously to givea tapered block structure and the polymers can be synthesized by eitherstep growth or chain growth methods.

The number average molecular weight of the polymer may be selected tosuit a particular application. As used herein, the term “number averagemolecular weight (Mn)” is given its ordinary meaning in the art andrefers to the total weight of the polymer molecules in a sample, dividedby the total number of polymer molecules in a sample. Those of ordinaryskill in the art will be able to select methods for determining thenumber average molecular weight of a polymer, for example, gelpermeation chromatography (GPC). In some cases, the GPC may becalibrated vs. polystyrene standards. In some cases, the number averagemolecular weight of the polymer is at least about 10,000, at least about20,000, at least about 25,000, at least about 35,000, at least about50,000, at least about 70,000, at least about 75,000, at least about100,000, at least about 110,000, at least about 125,000, or greater.

In an analyte-free state, the transition metal complex can interact withthe carbon-carbon multiple bond moiety of the conductivecarbon-containing material, for example, by a coordination of thetransition metal atom(s) with carbon atoms belonging to the conductivecarbon-containing material. The sensor can have a baseline level of ameasurable property in the analyte-free state.

When exposed to the analyte, at least a portion of the transition metalcomplex can bind to the analyte, e.g., to the electron-rich moiety suchas a carbon-carbon double bond, changing the nature and/or extent ofinteraction between the transition metal complex and the conductivematerial. This change is reflected in a change in the measurableproperty of the sensor; in other words, the sensor produces a measurableresponse when exposed to the analyte.

The sensor can provide high sensitivity to the analyte. For example, agaseous analyte such as ethylene can be detected at levels of less than100 ppm, less than 50 ppm, less than 20 ppm, less than 10 ppm, less than1 ppm, less than 5 ppm, less than 2 ppm, less than 1 ppm, less than 0.5ppm, or less. The sensor can also provide a linear response to analyteconcentration, such that an unknown concentration of the analyte can bedetermined based on the strength of the sensor response.

The transition metal complex can include a transition metal capable ofinteracting with a carbon-carbon multiple bond moiety. Such transitionmetals include but are not limited to Fe, Ru, Os, Co, Rh, Ir, Ni, Pd,Pt, Cu, Ag, or Au. The transition metal complex can include a transitionmetal capable of interacting with a carbon-carbon multiple bond moiety,coordinated by a multidentate ligand with coordinating atoms selectedfrom N and P, and optionally coordinated by an additional ligand L,which can be, for example, a carbon-carbon multiple bond moiety.

In some cases, the transition metal complex can have formula (I) or(II):

In formula (I), M can be a transition metal; each R¹, independently, canbe H, halo, alkyl, or haloalkyl; each R², independently, can be H, halo,alkyl, haloalkyl, or aryl; R³ can be H or alkyl; and L can be absent orrepresent a ligand. The ligand can in some cases be a η-2 carbon-carbonmultiple bond moiety or a carbon-heteroatom multiple bond moiety.

In formula (II), M can be a transition metal; each R⁴, independently,can be alkyl, haloalkyl, aryl, or trialkylsilyl. A can be—CH(R⁵)—X—CH(R⁵)— where X can be N or CH, and each R⁵, independently,can be H, halo, alkyl, or haloalkyl, or A can be —P(R⁶)₂—, where eachR⁶, independently, can be alkyl; and L can be absent or represent aligand.

Alkyl is a straight or branched hydrocarbon chain containing 1 to 16(preferably, 1 to 10; more preferably 1 to 6) carbon atoms, which can besubstituted or unsubstituted. The substituent can be a bond linking onegroup with an adjacent moiety or the conductive material. The alkylgroup can be optionally interrupted by —O—, —N(R^(a))—,—N(R^(a))—C(O)—O—, —O—C(O)—N(R^(a))—, —N(R^(a))—C(O)—N(R^(b))—, or—O—C(O)—O—. Each of R^(a) and R^(b), independently, can be hydrogen,alkyl, alkenyl, alkynyl, alkoxy, hydroxylalkyl, hydroxyl, or haloalkyl.In certain embodiments, the alkyl group can be optionally substitutedwith C₁₋₄ alkyl, C₂₋₄ alkenyl, C₂₋₄ alkynyl, C₁₋₄ alkoxy, hydroxyl,halo, amino, thio, alkylthio, arylthio, aralkylthio, acylthio, nitro,cyano, C₃₋₅ cycloalkyl, 3-5 membered heterocycloalkyl, monocyclic aryl,5-6 membered heteroaryl, C₁₋₄ alkylcarbonyloxy, C₁₋₄ alkyloxycarbonyl,C₁₋₄ alkylcarbonyl, or formyl.

In some embodiments, any of R¹, R², R³, R⁴, R⁵ or R⁶, independently, canbe covalently linked to another moiety, including the conductivematerial, for example, a carbon nanotube or a polymer.

The transition metal is a transition metal with one or more valencelevel d-electrons. In formulas (I) and (II), M can be Fe, Ru, Os, Co,Rh, Ir, Ni, Pd, Pt, Cu, Ag, or Au; in some cases, M can be Cu, Ag, orAu.

In some cases, the transition metal complex can have the formula:

where L can be absent or represents a ligand, each R¹, independently,can be H, methyl, or trifluoromethyl; each R², independently, can be H,methyl, trifluoromethyl, or phenyl; and R³ can be H or alkyl. L can beabsent, a thiol, an amine, a carbon-heteroatom multiple bond (forexample, MeCN) or a carbon-carbon multiple bond, e.g., an alkene, analkyne, or a carbon-carbon multiple bond moiety of a conductivecarbon-containing material. In some cases, each R¹ and each R² aretrifluoromethyl, R³ is H, and L is absent or ethylene or MeCN. Thetransition metal complex can be complex 1, copper(I)hydrotris[3,5-bis(trifluoromethyl)pyrazol-1-yl]borate:

1

In some cases, the transition metal complex can have the formula:

where L can be absent or represents a ligand, X can be N or CH, each R⁴,independently, can be alkyl (e.g., methyl, isopropyl, t-butyl),haloalkyl, or aryl (e.g., phenyl, pentafluorophenyl, or2,5-dimethylphenyl), and each R⁵, independently can be H, halo, alkyl(e.g., methyl, ethyl, propyl, isopropyl), or haloalkyl (e.g.,trifluoromethyl, perfluoropropyl). L can be absent, a thiol, an amine,or a carbon-carbon multiple bond, e.g., an alkene, an alkyne, or acarbon-carbon multiple bond moiety of a conductive carbon-containingmaterial.

In some cases, X can be N, each R⁵ can be perfluoropropyl, and each R⁴can be perfluorophenyl. In some cases, X can be CH, each R⁵ can bemethyl, and each R⁴ can be 2,5-dimethylphenyl.

In some cases, the transition metal complex can have the formula:

where L can be absent or represents a ligand; each R⁴, independently,can be trialkylsilyl (e.g., trimethylsilyl); and each R⁶, independently,can be alkyl (e.g., isopropyl, t-butyl) or haloalkyl. L can be absent, athiol, an amine, or a carbon-carbon multiple bond, e.g., an alkene, analkyne, or a carbon-carbon multiple bond moiety of a conductivecarbon-containing material. In some cases, each R⁶ is t-butyl and eachR⁴ is trimethylsilyl.

In some embodiments, the conductive material includes SWCNTs. Thetransition metal complex can interact with carbon-carbon double bondmoieties in the nanotube framework. When exposed to the analyte, theanalyte can bind to the transition metal complex, displacing it from thecarbon-carbon double bond moieties in the nanotube framework, causing achange in electrical properties (e.g., resistance) of the SWCNTs.

Optionally, the SWCNTs can be polymer-wrapped SWCNTs. For example,SWCNTs can be wrapped with a poly(thiophene). The poly(thiophene) caninclude pendant groups or side chains, which can bear a transition metalbinding group such as, for example, a thiol. The transition metalbinding group can interact with the transition metal complex. In thisway, the transition metal complex interacts with the conductive materialvia pendant groups on a polymer-wrapped SWCNT.

Optionally, the transition metal complex is covalently linked to theconductive material. Many conductive carbon-containing materials can befunctionalized; for example, carbon nanotubes can be functionalized witha variety of groups. For example, SWCNTs can be functionalized so as tobear a transition metal binding group such as, for example, a thiol. Thetransition metal binding group can interact with the transition metalcomplex. In this way, the transition metal complex interacts with theconductive material via covalent functional groups. In certainembodiments, the transition metal complex can be associated with thecarbon nanotube, for example, through a covalent or non-covalentinteraction. For example, a linker can be attached to the boron or otherpart of the ligand of the transition metal complex, which can becovalently bound to the carbon nanotube.

A carbon nanotube based system for ethylene sensing is illustratedschematically in FIG. 1. The ethylene sensitive material is an intimatemixture of SWCNTs with a copper(I) complex 1 based upon a fluorinatedtris(pyrazolyl) borate ligand, which is able to interact with thesurface of carbon nanotubes, thereby influencing their conductivity.Upon exposure to ethylene, 1 binds to ethylene and forms complex 2,which has a decreased interaction with the SWCNT surface. The result ofthis transformation is an increase in resistance of the SWCNT network.Complex 2 is one of the most stable copper-ethylene complexes known.See, e.g., H. V. R. Dias, et al., Organometallics 2002, 21, 1466-1473;and H. V. R. Dias, J. Wu, Eur. J. Inorg. Chem. 2008, 509-522; each ofwhich is incorporated by reference in its entirety. It is not easilyoxidized under ambient conditions and is stable in high vacuum. CompoundI has been employed in the detection of ethylene in fluorescenceschemes. See B. Esser, et al., Angew. Chem. Int. Ed. 2010, 49,8872-8875, which is incorporated by reference in its entirety.

Examples Fabrication

In a typical experiment 1 was ultrasonicated with SWCNTs in a mixture ofo-dichlorobenzene and toluene (2:3). Devices were prepared bydrop-casting the resulting dispersion onto glass slides withpre-deposited gold electrodes (as shown in FIG. 1). The experimentalsetup for sensing measurements is shown in FIG. 2 a. The device wasenclosed in a gas flow chamber (FIG. 2 b), with its electrodes connectedto a potentiostat. The analyte-gas mixture was produced in a gasgenerator, in which a stream of nitrogen gas was split into two parts,one of which as led through a flow chamber containing an ethylenepermeation tube or a piece of fruit. During a measurement, a continuousgas stream of constant flow rate, which could be switched betweendinitrogen and the analyte-dinitrogen mixture, was directed over thedevice. The results from exposing 1-SWCNT devices to low concentrationsof ethylene are shown in FIGS. 3 a-3 c. Ethylene concentrations of lessthan 1 ppm were detected, and measurements up to 50 ppm were performed.For many commodities, 1 ppm is the concentration at which ripeningoccurs at the maximum rate. Within the range of concentrations measured(0.5-50 ppm), a linear change in response was observed (see FIG. 3 c).

Devices made from pristine SWCNTs showed no response to the sameconcentrations of ethylene (see FIGS. 3 a-3 c). Further controls, inwhich [Cu(CH₃CN)₄]PF₆ or the sodium equivalent of 1 (Cu replaced by Na)were employed instead of 1 did not respond to ethylene either (seebelow). Employing the ethylene complex 2 resulted in device sensitivitytowards 20 ppm ethylene, however, the response amounted to only ˜25% ofthat of 1-SWCNT devices (see below). In optimizing the ratio of 1 toSWCNT we found that a large excess of 1 (ratio of 1 to SWCNT carbonatoms=1:6) resulted in the best sensitivity. Different types ofcommercially available SWCNTs were tested in the devices (see below).The best results were obtained with SWCNTs of small diameter, namelySWCNTs containing >50% of (6,5) chirality. The stronger curvature of thecarbon nanotube surface is believed to enhance the interaction between 1and the SWCNT.

Upon exposure to ethylene, a reversible increase in resistance wasobserved. This was ascribed to a mechanism as shown in FIG. 1, where theinteraction of 1 with the SWCNT surface induces doping of the nanotubes.When complexes 1 bind to ethylene, this doping effect is diminished, andhence an increase in resistance is measured. In order to rationalize theinteraction between 1 and the SWCNT surface, model calculations usingdensity functional theory were performed. The structure of complex 3,where the copper center in 1 is bound to the surface of a short segmentof a (6,5) SWCNT was optimized using the B3LYP functional with the6-31G* basis set for main group elements and LanL2DZ for Cu. Theoptimized structure of 3 is shown in FIG. 4. Steric interacions forcedone of the pyrazol rings of the ligand to be twisted in such a way thata trigonal planar coordination results for the Cu center. In anisodesmic equation, the binding strength of 1 to a (6,5) SWCNT fragment(3) was compared to the binding in 2. It was found that 2 is stronglyfavored over 3. Since reversible responses to ethylene were observed,the copper complexes 1 are believed to not completely dissociate fromthe SWCNTs, but bind the ethylene molecules in an associative fashion.

The Raman and IR spectra of 1-SWCNT are shown in FIG. 5. Uponintroduction of 1 into the SWCNT network a slight shift of the G and G′bands in the Raman spectrum to lower energies is observed, which can beindicative of p-type doping. See, e.g., A. Jorio, M. Dresselhaus, R.Saito, G. F. Dresselhaus, in Raman Spectroscopy in Graphene RelatedSystems, Wiley-VCH, Weinheim, Germany 2011, pp. 327 ff, which isincorporated by reference in its entirety. The IR spectrum of 1-SWCNTwas dominated by the C—F stretching modes of the ligand between1080-1260 cm⁻¹. The ν_(BH) shift was found at 2607 cm⁻¹. X-rayphotoelectron spectroscopy (XPS) measurements were used to confirm theratio of 1 to SWCNTs and to investigate the oxidation state of thecopper centers, which can undergo oxidation to copper(II). A ratio of1:22 was found for C_(SWCNT):Cu (based on the Cu 2p peak, see below fordata). In high resolution scans the characteristic pattern for copper(I)was observed, consisting of two peaks due to spin-orbit coupling at 932and 952 eV.

In order to investigate the sensing mechanism, field-effect transistor(PET) devices were prepared with 1-SWCNT or pristine SWCNT. A devicearchitecture with interdigitated Au electrodes (10 tun gap) on Si with300 nm SiO₂ was used. The source-drain potential was kept at a constantbias of 0.1 V, while the source-gate potential was scanned between +2and −20 V. A slight linear increase in conductance was observed towardsnegative gate voltages (see below for data), however, no strong gateeffect. This lack of a measurable shift in the turn-on voltage may bethe result of the fact that the charge injection (doping) differenceswere very small and/or due to device geometry and the nature of thenanotube network. In those cases where strong turn-on SWCNT 1-ETresponses are observed at negative gate voltages usually more highlyordered nanotube networks were employed. See, e.g., B. L. Allen, et al.,Adv. Mater. 2007, 19, 1439-1451; R. Martel, et al., Appl. Phys. Lett.1998, 73, 2447-2449; and S. Auvray, et al., Nano Lett. 2005, 5, 451-455,each of which is incorporated by reference in its entirety.

The system was used to compare the ethylene emission from a selection ofcommon fruits (banana, avocado, apple, pear, and orange). In theexperimental setup, the fruit was enclosed in the gas flow chamber asshown in FIG. 2, which allowed exposing the devices to fruit volatilesin the same way as to ethylene. The responses of 1-SWCNT devices to thedifferent fruits are shown in FIG. 6 a. The intensities are given inrelation to the response to 20 ppm ethylene and normalized to 100 gfruit. The largest responses were found for banana, followed by avocado,apple, pear, and orange. All fruit apart from orange showed ethyleneconcentrations above 20 ppm, which corresponded to emission ratesexceeding 9,600 mL/min. In order to follow the ripening and senescingprocess in these fruits, their ethylene emission was repeatedly measuredover several weeks (FIG. 6 b). Fruit can be classified into climactericand non-climacteric fruit according to respiration rate (release of CO₂)and C₂H₄ production pattern. Banana, avocado, apple, and pear belong tothe climacteric group, which is characterized by a large increase in CO₂and C₂H₄ production during ripening, while non-climacteric fruits, suchas orange, generally show low emission rates of these gases. Once theclimax (ripeness) is achieved, respiration and C₂H₄ emission decrease asthe fruit senesces. The climacteric rise during ripening was observed incase of the pear and avocado, which showed an increased ethyleneemission after the first week. For all other fruits and after the secondweek for the pear, measurements were conducted close to the maximumpoint of ripeness, and as a result the data reflects the senescence ofthe fruit with decreasing ethylene production rates for banana andapple. Two apples of the same kind and of similar ripeness werecompared, of which one was stored in a refrigerator (apple 1), whileapple 2 was kept at room temperature. As anticipated, apple 2 senescedfaster at room temperature, and hence its ethylene production decreasedat a quicker pace than for apple 1. The orange as a non-climactericfruit showed an overall low emission rate of ethylene.

In order to assess the selectivity of our sensory system, responses of1-SWCNT devices to several solvents (75-200 ppm concentrations) asrepresentatives of functional groups were measured, as well as toethanol and acetaldehyde, which occur as fruit metabolites. The resultsare shown in FIG. 7 in comparison to the response to 50 ppm ethylene andto pristine SWCNTs.

Significantly high responses were observed towards acetonitrile, THF,and acetaldehyde, while all other solvents had only small effects.However, considering the concentrations of these compounds the responseswere smaller in magnitude than the response to ethylene (50 ppm ethylenevs. 100 ppm acetonitrile, 200 ppm THF or 75 ppm acetaldehyde). Thesensitivity of 1-SWCNT devices towards these analytes was notsurprising, as they are able to bind to the copper center in 1 via thenitrile group (acetonitrile), the ether group (THF), or the oxygen ofacetaldehyde.

The concentrations required for fruit ripening lie in most cases between0.1 and 1 ppm, and hence in storage facilities the ethylene level is tobe kept below those thresholds. The sensory system consisting of 1 andSWCNTs showed good responses down to 1 ppm of ethylene. Sensitivity canbe improved by increasing the surface area and porosity of the SWCNTnetwork structure. In order to achieve this 5 weight-% cross-linkedpolystyrene beads of 0.4-0.6 μm diameter was added to the mixture, fromwhich devices were prepared. The responses of the resulting 1-PS-SWCNTdevices to ethylene concentrations of 0.5, 1, and 2 ppm are shown inFIG. 8. A 1.3-2.2 fold increase in sensitivity was observed, which wasattributed to an increased surface area of the SWCNT network andpossibly an increase in the local ethylene concentration in the deviceby partitioning into the polystyrene beads—in other words, apreconcentration effect.

Materials and Synthetic Manipulations.

Synthetic manipulations were carried out under an argon atmosphere usingstandard Schlenk techniques. [CF₃SO₃Cu]₂.C₆H₆ was purchased from TCIAmerica, hydrotris[3,5-bis(trifluoromethyl)pyrazol-1-yl]borato sodium(Na[HB(3,5-(CF₃)₂-pz)₃]) was prepared following a literature procedure(H. V. R. Dias, et al., Inorg. Chem. 1996, 35, 2317-2328, which isincorporated by reference in its entirety). Single-walled carbonnanotubes were purchased from SouthWest Nano Technologies (SWeNT® SG65,SWeNT® SG65-SRX, SWeNT® SG76, and SWeNT® CG100) or from Unidym (HIPCO®Super Purified). Cross-linked polystyrene particles (0.4-0.6 μmdiameter) were purchased from Spherotech and transferred from water intotoluene. Dry toluene was purchased from J. T. Baker. All other chemicalswere purchased from Sigma Aldrich and used as received. NMR spectra wererecorded on Bruker Avance-400 spectrometers.

Synthesis of 1

8 mg (15.9 μmol) [CF₃SO₃Cu]₂.C₆H₆ were dissolved in 3 mL dry, degassedtoluene. 17 mg (43.5 μmol)hydrotris[3,5-bis(trifluoromethyl)pyrazol-1-yl]borato sodium(Na[HB(3,5-(CF₃)₂-pz)₃]) were added, and the mixture was stirred for 14h at r.t. The reaction mixture was filtrated through a syringe filter toreceive a colorless solution of 1 with a concentration of ˜6 μmol/mL (6mM).

The exact concentration of 1 was determined in the following way: Asmall amount of the solution was purged with ethylene for 20 min. Thesolvent was then evaporated, and the concentration of 1 determined byNMR spectroscopy using benzene as a reference for integration.

Preparation of 1-SWCNT.

0.50 mg (41.6 mol carbon) of SWCNTs were suspended in 0.8 mL dryo-dichlorobenzene, and 1.16 mL (6.9 μmol) of a 6 mM solution of 1 intoluene were added. The mixture was sonicated at 30° C. for 30 mM. Theresulting black dispersion of 1-SWCNT was used to prepare devices.

Preparation of 1-PS-SWCNT.

0.50 mg (41.6 μmol carbon) of SWCNTs were suspended in 0.8 mL dryo-dichlorobenzene, and 1.16 mL (6.9 μmol) of a 6 mM solution of 1 intoluene as well as 2.4 μL of a suspension of cross-linked polystyreneparticles in toluene (5 μg/mL) were added. The mixture was sonicated at30° C. for 30 min. The resulting black dispersion of 1-PS-SWCNT was usedto prepare devices.

Device Preparation.

Glass slides (VWR Microscope Slides) were cleaned by ultrasonication inacetone for 10 min, and after drying they were subjected to UV radiationin a UVO cleaner (Jelight Company Inc.) for 3 min. Using an aluminummask, layers of chromium (10 nm) and gold (75 nm) were deposited leavinga 1 mm gap using a metal evaporator purchased from Angstrom Engineering.Volumes of 1 μL of the dispersion of 1-SWCNT was drop-cast in betweenthe gold electrodes followed by drying in vacuum until a resistance of1-5 kΩ was achieved.

Sensing Measurements.

Devices were enclosed in a homemade Teflon gas flow chamber for sensingmeasurement (see FIGS. 2 a-2 b). The gold electrodes of the device werecontacted with connections to the outside of the gas flow chamber, andtwo ports on opposite sides of the chamber allowed to direct acontinuous gas flow through the chamber. The low concentration gasmixtures were produced using a KIN-TEK gas generator system. A traceamount of analyte emitted from a permeation tube is mixed with anitrogen stream (oven flow), which can be further diluted with nitrogen(dilution flow). For ethylene, refillable permeation tubes were used,while for the solvents calibration measurements were performed byplacing the solvent in the oven flow for set amounts of time. For fruitmeasurements, the fruit was placed in a flow chamber, through which the“oven flow” was directed, which was then further diluted with nitrogen.

Electrochemical measurements were performed using an AUTOLAB instrumentfrom Eco Chemie B.V. A constant bias voltage of 0.1 V was applied acrossthe device, while current vs. time was measured. During the measurementthe volume of gas flow over the device was held constant and switchedbetween nitrogen and analyte/nitrogen.

FET Measurements.

As a substrate for FET measurements, a piece of silicon with a 300 nmSiO₂ insulating layer onto which Au electrodes had been deposited, waschosen. Interdigitated electrodes with a 10 μm gap were used. Analogousto the preparation of the devices for amperometric sensing measurements,dispersions of 1-SWCNT and of pristine SWCNTs were drop-cast betweenthese electrodes. For the measurements, the device was enclosed in ateflon chamber analogous to FIG. 2B with an additional electrode tocontact the Si bottom gate. The source-gate potential was swept from +2V (+5 V in the case of 1-SWCNT) to −20 V at a constant source-drain biasof 0.1 V and the chamber was flooded with nitrogen during themeasurement. The source-drain current as well as the gate leakagecurrent were recorded (FIGS. 9 a-9 d).

Testing of different SWCNT Types and Control Experiments.

While optimizing sensitivity of the devices to ethylene, different typesof SWCNTs were tested. In FIG. 10 (left) are shown the relativeresponses of devices made from different 1-SWCNT dispersions. Theresults of control experiments, in which dispersions of 2-SWCNT, 4-SWCNT(see below for structure of 4) and SWCNTs with [Cu(CH₃CN)₄]PF₆ were usedto prepare devices are shown on the right in FIG. 10.

Fruit Information.

Fruit of the following types and weight was purchased from a Farmer'smarket: Banana (Cavendish)—142.5 g; Avocado (Hass)—170.7 g; Apple 1(Macintosh)—119.1 g; Apple 2 (Macintosh)—111.3 g; Pear (Cornice)—246.1g; Orange (Navel)—265.0 g.

Raman Measurements, IR Measurements, and XPS Data.

IR spectra were recorded on a SMART iTR purchased from ThermoScientific. The sample was dropcast onto a KBr card, and the spectrummeasured in transmission mode. Raman spectra were measured on a HoribaLabRAM HR Raman Spectrometer using excitation wavelengths of 785 nm and532 nm. The samples were dropcast onto SiO₂/Si substrates for themeasurement. XPS spectra were recorded on a Kratos AXIS Ultra X-rayPhotoelectron Spectrometer. The samples were drop-cast onto SiO₂/Sisubstrates for the measurements. As the copper complex 1 is airsensitive, it was drop-cast under argon and the exposure to air was keptminimal (<2 min) during the transfer into the XPS instrument. In thecase of 1 and 2 sample charging was observed and a charge neutralizerwas used. The resulting shift in energy was compensated by calibratingusing the F is peak at 687 eV. FIG. 11 shows results of the XPSmeasurements.

Isodesmic Equation.

The isodesmic equation that allows comparing the binding strength of 1to ethylene or a SWCNT is:

Electronic and Zero-Point Vibrational Energies.

Electronic energies (C_(o)), zero point vibrational energies (ZPVE),total energies (E_(total)), and free energies G for all calculatedstructures (local minima) of the isodesmic equation (B3LYP/6-31G* for C,H, B, F, N, LanL2DZ for Cu) are shown in Table 1.

TABLE 1 Compound ε₀ [hartrees] ZPVE [hartrees] E_(total) [kcal/mol] 2−3000.31041 0.28383 −1882546.7 3 −7126.899656 — — Ethylene −78.587460.05123 −49282.3 (6,5) SWCNT −4205.29893 0.92740 −2638285.2 fragment

Polymer-Wrapped SWCNTS.

FIG. 12 illustrates a polythiophene-wrapped SWCNT having pendant groupsthat bind to a transition metal complex. When exposed to an analyte,e.g., ethylene, the analyte binds to the transition metal complex,displacing it from the pendant group. The SWCNT has differingresistances in these two states.

Polythiophenes for wrapping SWCNTs, PT1, PT2, PT3, and PT4, are shownbelow:

Polymer-wrapped SWCNTs were prepared by combining a polythiophene (PT)with SWCNT in CHCl₃ and sonicating. The mixture was centrifuged and thesupernatant isolated; material was then precipitated with ethanol,providing polythiopene-wrapped SWCNT (PT/SWCNT). These were suspended inCHCl₃ and a solution of copper complex 1 in toluene was added, affordingPT/SWCNT/1 complexes. These were spin coated over gold electrodes formeasurements.

FIG. 13 shows relative responses of a PT1/SWCNT/1 device to lowconcentrations of ethylene. Reversible responses to low concentrationsof ethylene were observed, with sensitivity down to 100 ppm of ethylene.PT1/SWCNT devices without any transition metal complex showed noresponse even to 6000 ppm ethylene.

Covalently modified SWCNTs. FIG. 14 illustrates a covalently modifiedSWCNT having functional groups that bind to a transition metal complex.When exposed to an analyte, e.g., ethylene, the analyte binds to thetransition metal complex, displacing it from the functional group. TheSWCNT has differing resistances in these two states.

FIG. 15 illustrates the functionalization of SWCNTs: SWCNTs werecombined with S-(2-azidoethyl)thiophenol (1 equiv. per carbon) ino-dichlorobenzene at 160° C. for 2 days to provide modified SWCNTs.Devices were prepared by combining modified SWCNTs and 1 ino-dichlorobenzene and sonicating, then dropcasting the resultingcomplexes between gold electrodes. FIG. 16 shows relative responses ofsuch a device to low concentrations of ethylene. Reversible responses tolow concentrations of ethylene were observed, with sensitivity to lessthan 100 ppm of ethylene.

Ethylene Sensors by Abrasion

Preparation of a 1-SWCNT Pellet.

94 mg (0.187 mmol) [CF₃SO₃Cu]₂.C₆H₆ were dissolved in 30 mL dry,degassed toluene. 200 mg (0.311 mmol)hydrotris[3,5-bis(trifluoromethyl)pyrazol-1-yl]borato sodium(Na[HB(3,5-(CF₃)₂-pz)₃]) were added, and the mixture was stirred for 15h at r.t. The reaction mixture was filtered under argon to yield acolorless solution of 1 with a concentration of ˜13 μmol/mL (13 mM), asdetermined by NMR. 31.7 mg (2.64 mmol carbon) of SWCNTs were added tothe solution, and the resulting mixture was sonicated at 30° C. for 30mM under argon. The resulting black dispersion was evaporated to drynessin vacuo yielding 207 mg of a black powder.

Preparation of a 2-SWCNT Pellet.

370 mg (0.70 mmol) [CF₃SO₃Cu]₂.C₆H₆ were dissolved in 38 mL dry,degassed toluene. 1 g (1.55 mmol)hydrotris[3,5-bis(trifluoromethyl)pyrazol-1-yl]borato sodium(Na[HB(3,5-(CF₃)₂-pz)₃]) were added, and the mixture was stirred for 17h at r.t. Subsequently, ethylene was bubbled through the solution for 40mM. The solution was then stirred for 4 h in an ethylene atmosphere atr.t. Solids were removed by filtration through a glass frit and solventwas removed from the resulting solution. 497 mg (0.7 mmol) of 2 wereobtained as a white powder. 125 mg of 2 were mixed with 25 mg SWCNTs byball-milling yielding a black powder.

Sensor Fabrication by Drawing and Sensing Measurement.

The black powder of 1+SWCNT or 2+SWCNT was subsequently compressed intoa pellet and sensors were fabricated by drawing with the pellet betweentwo gold electrodes on paper. Sensing measurements were performed asdescribed above. The complex formed with ethylene is represented below.

The response of sensor fabricated by drawing with a pellet of Cu(I)scorpionate 2 and SWCNTs to ethylene can be seen at FIGS. 17-19. Thesensing response of devices fabricated by abrasion with pristine SWCNTs,SWCNTs+KMnO₄ and SWCNTs+1 on HP multipurpose paper to 500 ppm ethylenecan be seen at FIG. 20. The sensing response of devices fabricated byabrasion with SWCNTs+1 and pristine SWCNTs on the surface of weighingpaper to 40 ppm ethylene can be seen at FIG. 21.

Generation of Sensing Material Via Spray-Drying

Spray-drying of a mixture of 1 and SWCNTs can potentially lead to bettermixing of both components and thus potentially a higher sensingperformance. This is most relevant for the abrasion fabrication methodabove.

Material Preparation.

SWCNTs were suspended in dry o-dichlorobenzene (1.6 mL per mg ofSWCNTs), and ⅙ equivalents of 1 in toluene were added to obtain asuspension containing 0.3 wt % total solid material in 1:1o-dichlorobenzene/toluene. The mixture was sonicated at 30° C. for 30min. The resulting black suspension was subjected to spray-drying at anozzle temperature of 210° C. in a nitrogen atmosphere. A highly viscousproduct was obtained.

Device Preparation.

Gold (100 nm) was deposited onto pieces of paper using a shadow mask ina metal evaporator purchased from Angstrom Engineering. The resultingdevices contained 9 separate working electrodes and one shared counterelectrode at a gap size of 1 mm. The previously obtained materialcontaining 1-SWCNT as well as residual solvent was applied to the gap ofthe device using a spatula.

Sensing Measurements.

The device was enclosed in a homemade Teflon gas flow chamber andconnected to an array potentiostat via an edge connector and breadboard.A continuous flow of gas was applied to the device in the chamber usinga KIN-TEK gas generator system. A trace amount of analyte emitted from apermeation tube is mixed with a nitrogen stream (oven flow), which canbe further diluted with nitrogen (dilution flow). For ethylene,refillable permeation tubes were used. A graph of the measurements canbe seen at FIG. 22.

Polymer Coating of Sensor Devices, Composites

The current lifetime of our ethylene sensors is currently ca. 2 weeksand we would like to increase it. Furthermore, some coatings could“shield” the sensor from moisture while being permeable to ethylene.Lastly, coatings could have a preconcentrator effect.

Polymer coated devices as described below have been prepared and tested.However, a response to ethylene with that type of setup has not beenachieved due to technical difficulties. Preparation of polymer coateddevices:

Device Preparation.

Glass slides (VWR Microscope Slides) were cleaned by ultrasonication inacetone for 10 min, and after drying they were subjected to UV radiationin a UVO cleaner (Jelight Company Inc.) for 3 min. Using a stainlesssteel shadow mask, layers of chromium (10 nm) and gold (100 nm) weredeposited resulting in 14 working electrodes and 1 shared counterelectrode with a 1 mm gap using a metal evaporator purchased fromAngstrom Engineering. Volumes of 1 μl, of the dispersion of 1-SWCNT wasdrop-cast in between the gold electrodes followed by drying in vacuumuntil a resistance of 1-5 kΩ was achieved.

A solution of a polymer in dichloromethane was prepared by adding 10 mgof the polymer to 1 mL of DCM, followed by sonication. 2 times 1 μL ofthe solution was drop-cast onto the 1-SWCNT material of the sensor. Twodevices each were prepared with the following polymers: polyethylene,polystyrene, poly(ethylene oxide), polyvinylidene fluoride, Nafion, andPoly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene].

Sensing Measurement.

Subsequently, the glass slide with 14 devices was enclosed in a homemadeTeflon gas flow chamber and connected to an array potentiostat via anedge connector and breadboard. See FIG. 23. A continuous flow of gas wasapplied to the device in the chamber using a KIN-TEK gas generatorsystem. A trace amount of analyte emitted from a permeation tube ismixed with a nitrogen stream (oven flow), which can be further dilutedwith nitrogen (dilution flow). For ethylene, refillable permeation tubeswere used.

Array Sensors

Combining different sensors into a sensor array can have severaladvantages. Reproducibility can be improved by signal averaging overseveral sensors of the same type, life-time can be improved by creatingadditional redundancies, i.e. if one sensor fails, other sensors canstill work. Additionally, sensors of different types can be combined toimprove selectivity of the sensor. For this goal, ethylene sensors thatuse different sensing materials can be combined. The different materialswill likely lead to different reaction to interferents. Also, sensorsthat are designed specifically to react with interferents (e.g. water,alcohols, aldehydes, ketones, esters, hydrocarbons etc.) can be includedto correctly observe the response to these analytes and thus avoid falsepositives.

Material Preparation.

Ball-Milling: SWCNTs were mixed with a selector, such as Ag(OTf) orPd(OCOCF₃)₂ at a weight ratio of 5:1 selector to SWCNT and subjected toball milling. The obtained material was compressed into a pellet.

Spray-Drying: SWCNTs were mixed with a selector, such as Ag(OTf) orPd(OCOCF₃)₂. 100 mL toluene were added and the mixture was sonicated for5 minutes as well as throughout the spray-drying process. The suspensionwas spray-dried at a nozzle temperature of 180° C. in a nitrogenatmosphere yielding a black powder. The powder was compressed into apellet.

To obtain 1-SWCNT, SWCNTs were suspended in dry o-dichlorobenzene (1.6mL per mg of SWCNTs), and ⅙ equivalents of 1 in toluene were added toobtain a suspension containing 0.3 wt % total solid material in 1:1o-dichlorobenzene/toluene. The mixture was sonicated at 30° C. for 30min. The resulting black suspension was subjected to spray-drying at anozzle temperature of 210° C. in a nitrogen atmosphere. A highly viscousproduct was obtained.

Device Preparation.

Gold (100 nm) was deposited onto pieces of paper using a shadow mask ina metal evaporator purchased from Angstrom Engineering. The resultingdevices contained 9 separate working electrodes and one shared counterelectrode at a gap size of 1 mm. The material containing 1-SWCNT as wellas residual solvent was applied to the gap of the device using aspatula. Other materials were applied to the substrate by abrasion ofthe respective material pellet.

Sensing Measurement.

Subsequently, the devices were enclosed in a homemade Teflon gas flowchamber and connected to an array potentiostat via an edge connector andbreadboard. A continuous flow of gas was applied to the device in thechamber using a KIN-TEK gas generator system. A trace amount of analyteemitted from a permeation tube is mixed with a nitrogen stream (ovenflow), which can be further diluted with nitrogen (dilution flow). Forethylene, refillable permeation tubes were used. Under the investigatedconditions, 1-SWCNT based sensors showed a response to ethylene whilethe other materials did not show a response (FIG. 24). THF on the otherhand led to a response of all sensors in the array (FIG. 25).

Thus, while a single 1-SWCNT based sensor might not be able todistinguish the two analytes, the presented array allows thisdistinction.

Other embodiments are within the scope of the following claims.

What is claimed is:
 1. A sensor comprising: a conductive material comprising a carbon-carbon multiple bond moiety, the conductive material being in electrical communication with at least two electrodes; and a transition metal complex capable of interacting with a carbon-carbon multiple bond moiety.
 2. The sensor of claim 1, wherein the conductive material includes a carbon nanotube.
 3. The sensor of claim 2, wherein the transition metal complex is capable of forming a stable complex with ethylene.
 4. The sensor of claim 3, wherein the transition metal complex is associated with the carbon nanotube by coordination of the transition metal to the carbon-carbon multiple bond moiety.
 5. The sensor of claim 3, wherein the transition metal complex is associated with the carbon nanotube by a covalent link between the carbon nanotube and a ligand of the transition metal complex.
 6. The sensor of claim 3, wherein the transition metal complex is associated with the carbon nanotube by a polymer which is non-covalently associated with the carbon nanotube.
 7. The sensor of claim 1, wherein the transition metal complex is bound to the carbon-carbon multiple bond moiety of the conductive material.
 8. The sensor of claim 1, wherein the transition metal complex has formula (I):

wherein: M is a transition metal; each R¹, independently, is H, halo, alkyl, or haloalkyl; each R², independently, is H, halo, alkyl, haloalkyl, or aryl; R³ is H or alkyl; and L is absent or represents a ligand; or the transition metal complex has formula (II):

wherein: M is a transition metal; each R⁴, independently, is alkyl, haloalkyl, aryl, or trialkylsilyl; A is —CH(R⁵)—X—CH(R⁵)— wherein X is N or CH, and each R⁵, independently, is H, halo, alkyl, or haloalkyl; or A is —P(R⁶)₂—, wherein each R⁶, independently, is alkyl; and L is absent or represents a ligand.
 9. The sensor of claim 8, wherein the transition metal complex has the formula:

wherein: each R¹, independently, is H, methyl, or trifluoromethyl; each R², independently, is H, methyl, trifluoromethyl, or phenyl; R³ is H or methyl; and L is absent, a thiol, or a carbon-carbon multiple bond.
 10. The sensor of claim 1, wherein the transition metal complex and the carbon-carbon multiple bond moiety are mixed with a polymer.
 11. The sensor of claim 10, wherein the carbon-carbon multiple bond moiety is a carbon nanotube and the polymer is a polymer bead.
 12. A method of sensing an analyte, comprising: exposing a sensor to a sample, the sensor including: a conductive material comprising a carbon-carbon multiple bond moiety, the conductive material being in electrical communication with at least two electrodes; and a transition metal complex capable of interacting with a carbon-carbon multiple bond moiety; and measuring an electrical property at the electrodes.
 13. The method of claim 12, wherein the sample is a gas.
 14. The method of claim 12, wherein the electrical property is resistance or conductance.
 15. The method of claim 12, wherein the analyte is ethylene.
 16. The method of claim 12, wherein the conductive material includes a carbon nanotube.
 17. The method of claim 16, wherein the transition metal complex is capable of forming a stable complex with ethylene.
 18. The method of claim 17, wherein the transition metal complex is associated with the carbon nanotube by coordination of the transition metal to the carbon-carbon multiple bond moiety.
 19. The method of claim 17, wherein the transition metal complex is associated with the carbon nanotube by a covalent link between the carbon nanotube and a ligand of the transition metal complex.
 20. The method of claim 17, wherein the transition metal complex is associated with the carbon nanotube by a polymer which is non-covalently associated with the carbon nanotube.
 21. The method of claim 12, wherein the transition metal complex is bound to the carbon-carbon multiple bond moiety of the conductive material.
 22. The method of claim 12, wherein the transition metal complex has formula (I):

wherein: M is a transition metal; each R¹, independently, is H, halo, alkyl, or haloalkyl; each R², independently, is H, halo, alkyl, haloalkyl, or aryl; R³ is H or alkyl; and L is absent or represents a ligand; or the transition metal complex has formula (II):

wherein: M is a transition metal; each R⁴, independently, is alkyl, haloalkyl, aryl, or trialkylsilyl; A is —CH(R⁵)—X—CH(R⁵)— wherein X is N or CH, and each R⁵, independently, is H, halo, alkyl, or haloalkyl; or A is —P(R⁶)₂—, wherein each R⁶, independently, is alkyl; and L is absent or represents a ligand.
 23. The method of claim 22, wherein the transition metal complex has the formula:

wherein: each R¹, independently, is H, methyl, or trifluoromethyl; each R², independently, is H, methyl, trifluoromethyl, or phenyl; R³ is H or methyl; and L is absent, a thiol, or a carbon-carbon multiple bond.
 24. A method of making a sensor comprising: forming a complex including a conductive material comprising a carbon-carbon multiple bond moiety, and a transition metal complex capable of interacting with a carbon-carbon multiple bond moiety; and placing the conductive material in electrical communication with at least two electrodes.
 25. The method of claim 24, wherein the transition metal is copper.
 26. The method of claim 24, wherein the electrodes are gold.
 27. The method of claim 24, wherein the sensor is configured to sense ethylene.
 28. The method of claim 24, wherein the complex is Cu(I) scorpionate.
 29. The method of claim 24, wherein placing the conductive includes drop-casting a solution of the transition metal complex and a polymer onto the at least two electrodes.
 30. The method of claim 29, wherein the polymer can be selected from the group consisting of a hydrophobic polymer, a fluorinated polymer, a conjugated or partially conjugated polymer and combinations thereof.
 31. The method of claim 24, further comprising combining the complex mixture with a selector.
 32. The method of claim 24, wherein the selector includes a transition metal salt.
 33. A method of making a sensor comprising: forming a complex including a conductive material comprising a carbon-carbon multiple bond moiety, and a transition metal complex capable of interacting with a carbon-carbon multiple bond moiety; spray drying the complex at a temperature to obtain a viscous conductive material; and placing the viscous conductive material in electrical communication with at least two electrodes.
 34. The method of claim 33, wherein the temperature is between 100 and 210° C.
 35. The method of claim 33, wherein the spray drying takes place in an inert atmosphere. 